CSS
Cascading Style Sheets (CSS) is a means of separating the content of an html document from the style and layout of that document.
That means : You only need ONE file to display your whole site design throughout your whole website.
The visitors to your website only need to download this file once. This file is automatically used over and over again by the browser to display the design for every single page the user visits.
If you think about the speed of light your website will be ready loading, CSS makes your website lighter. It will also save a lot of bandwidth.
In future, you will be able to save 80% of your time when doing a redesign of your website.